1. Field of the Invention
The invention relates in general to an image color correction technique, and more particularly, to an estimation method of white-point shift for white balance suitable for use to correct white balance of a color image sensing apparatus.
2. Related Art of the Invention
Along with the development of image processing technique, sensing photographic technique and the variation of information communication, the image photographic device has become a necessary information appliance of our daily lives. For example, the actual image can be photographed and replayed later. Lately, the highly developed digital image photographic technique that records the image in pixel units, and stores and displays the image in digital form has advanced the popularity of digital cameras or digital camcorders as the color image is the information that we cannot avoid for our lives.
A color image is basically mixed by three primary colors, that is, red (R), green (G) and blue (B). Regardless the blackbody radiation factor, a white point is formed of a mixture of isometric R, G and B lights as shown in FIG. 1. When R, G and B lights are weighted differently, different color is produced. In FIG. 1, the line between the white point (1,1,1) and the black point (0,0,0) indicates the gray level.
When the characteristic of actual image sensing device is taken into account, the white point is deviated from that as illustrated in FIG. 1. The hardware design of the sensing device has various color responses. Further, under different light sources such as natural light source or artificial light source, different color responses are resulted under the influence of color temperature. For example, when the color temperature of the light source is as low as about 2850° K, the color approaches to red and white. In contrast, when the color temperature of the light source is increased up to about 8500° K, the color tends to be blue. The color differences affect the image color fidelity.
Therefore, how to locate the white point under various color temperatures and how to estimate tone and white balance point shifts are essential for color calibration.
Based on the color principles, colors have been illustrated according to the characteristics of a variety of color coordinates. Currently, in addition to the RGB coordinate, transformation matrix is readily applied between other commonly seen color coordinates such as CYM, CYMK, HIS, HSV, YCbCr, YUV and YIQ coordinates.
To resolve the problem of white balance, the YCbCr coordinate (Y is the brightness component, Cb is the blue component, Cr is the red component) is typically used to perform color analysis because a better compression effect can be obtained for typical JPEG pictures by the YCbCr illustration. As shown in FIG. 2, the white point is deflected into a curved surface in accordance with the variation of color temperature. For a particular brightness, the white point varies along a curve. For different brightness, the curvature of the curve is different. Therefore, the white point thus constructs an irregular curved surface in the YCbCr coordinated. However, it is difficult to transform the curved surface of white point in the YCbCr coordinate into a mathematic formula.